Ultrasonic compressional wave transmission



Feli- 24, 1948 H. B. BRIGGs Er A1. 2,436,377

ULTRASONI C. COMPRES S IONAL WAVE TRANSMI SS ION Filed Dec. 27, 1943 ArroR/vsv Parent@ Feb. 24, ,194s

ULTRASONIC COMPRESSIONAL y vTRANSMISSION Maplewood, and Warren P. Mason, -West noma n. Bam. Chatham, John n. Jonatan; Y

Orange, N. J., assiznora to Bell Telephone. Laboratories Incorporated, New York, N. Y., a corporation .of New York f Application December 27, 1943, `Serial No. 515,730 ,l

Y Y 1 This invention relates to" ultrasonic compressional wave transmission and has for its object the provision of a method for transmitting highl power compressional waves.

It is known that the phenomenon of cavitation imposes a limit on the amount of power which may be transmitted in a liquid by a transducer regardless of the capability of such transducers.

One form of transducer in common use consists of an array of piezoelectric crystals which respond to electrical excitation to produce compressional Waves in a surrounding medium such 3 Claims. (Cl. 177-386) as sea water; but it has been found that sea water f ing between such surrounding liquid and the sea water a diaphragm having a much greater area than the eiective area of the faces of the transducers so that the power transmitted thereby per unit of area is comparatively' small. In this way the total Power transmitted maybe greatly increased. However, even in this case, the power transmitted by the transducer has to be limited a reasonable amount below the point at whichI oisuch other liquid but will provide a means for the rapid evolution of gas and thus enhance the false cavitation effect. With such a liquid a transducer may be worked at a higher power intensity much nearer the limit than heretofore since an occasional operation beyond the limit will not endanger the transducers.

The'present inventors have found that there is a time delay between the application of power to the transducer and the establishment of the phenomenon of cavitation. Unlike certain electricalphenomena such as the breakdown oi' insulation when the dielectric strength thereof has been ex ceeded, there is here a deiinite time delay in the appearance of the phenomenon. Also unlike the low governing the heating of a coil of wire where the heating is proportional to the product of the intensity of the current andthe time of application thereof, a peculiar relation exists between intensity of power transmitted and the period required under such conditions for cavitation to be come established. This peculiar fact is employed by the present inventors to transmit a high concentration of power-very much more than can be transmitted after cavitation has been established by limiting such transmission to periodsshorter than the period required for the establishment of cavitation. IIt has been found that cavitation will begin so as to avoid destruction of Y into two regionsone known as false cavitationy and the other known as true cavitation. False cavitation is the evolution of gas and in a medium wherein gas may be evolved easily generally precedes true cavitation as the power is increased.

When gas is evolved the collapse of the voids is nowhere near as violent as in true cavitation and therefore the destructive eiect is not as great. A mixture of liquids has therefore been devised in which one liquidhaving a high vapor pressure will not through such admixture with another adverscly affect the otherwise excellent properties high power signals may thus be transmitted periodieally without undue limitation from cavitation.

A feature of the invention is the use of an ultrasonic compressional wave transmitting device2 for short periods at a power level above that which may be used continuously. By this means an ultrasonic direction and ranging system may be employed in a vessel or other hulllmoving through water .at such a high rate of speed that considerable acoustic noise is developed. Due to this noise a high energy density must be used ii' the direction and ranging system is to be eective for any considerable distance. Since in such a system signals in the form of very short impulses are transmitted the power used may be ex- I tremely high.

The drawings consist of a single sheet having two iigures as follows:

Fig. 1 is a cross-sectional view of a crystal transducer which may be suitably employed; and

Fig. 2 is a set of graphs showing the relation between energy density which may be applied to such a transducer and the pulse length in milliseconds, for various liquids.

The electromechanical transducer shown in Fig. 1 comprises a casing I of steel or other rigid asada?? such as i'llters and delay networks used when such a transducer is connected asa vprism array. The

- device is shown as submerged in water I2 and the' vertical broken lines to the righ of the pc rubber cap represent a compressional wave as being transmitted. Available data indicate that cavitatl'on results from the collecting of minute bubbles of air or vapor into larger bubbles that can be seen and which aiect materially the'acousti'c outputv of a transducer. This collecting or joining together of the minute bubbles is a process that takes time. TheV rapidity withwhich itoccurs also depends onhow much energyis'put into the liquid. Hence, if a certain amount of energy is required to produce cavitation under steady state conditions, it requires a larger amount of energy to produce cavitation for a relatively long pulse, followed by a period when no energy is sent4 out; and a stillgreater energy for a shorter pulse.

Fig. -2 shows experimental results obtained for Y degassed acetylated castor oil, 'graph i4, trans- Vformel' oil, graph I5, and for non-degassed acetylated castor oil, graph I6. For the degassed electrical grade castor `oil ,.graph I3, the power went from 3 watts per square centimeter of crystal face to 6 wattsper square centimeter from the steady state to 10 millisecond pulses. If this had been a plain heating effect following .the usual law the rise would have been greater, for .the pulse was applied for 10 milliseconds and then there was an oil period of 990 milliseconds vso that only 2 per cent of the energy was put in the liquid for the pulsing case as compared with '2.5 watts per square centimeter before going into the water.

The graph l0 for ynon-degassed acetylated castor oil (which issimilar to vsea water) shows that the water would take this power without cavitation and main body of water.

Heretofore, projectors have been designed on a basis of keeping the energy inthe water below Va-watt per square centimeter (the steady state cavitation value) 'A projector delivering 2 to 2.5 wattsper square centimeter to theI water represents an increase of 6 or 'l to 1 and thus represents an appreciable increase in the distance that can be obtained by the pulsing method. Also for a system to be employed where the vessel is travcling through the water at 45. knots and hence is making considerable acoustic noise, it is impera- 'tive to get back inthe echo all the energy postransmit it to the lsible in order to work throughthisl noise.l Thel factor of I-to 1 in energy. ratio (8.5 decibels) allows the echo ranging system to override the noise to a distanceof 2.35 times the limiting distance for a projector designed'on the previous steady'state. basis.

What is claimed is: .v

1. The method of increasing the eective range oi' sound ranging'by compressional waves in sea.

water, consisting of the transmission of highv I failure of an electromechanical transducer operated at a power levelabove that which will prothe steady state. Here the rise in power, instead of vbeing 100 times (as it should be for a heating enect) was only 2 times.

The graphs show that liquids do not all act the same, transformer oil, for example, showing smaller increase than acetylated castor oil. While no graph is shown for water, the nondegassed sea water that will be outside the projector gives labout the same result as the nondegassed acetylated castor oil, graph I6.

. Hence, if a projector such as that shown in duce the phenomenon ofcavitation which consists of limiting the periods of operation thereof to periods less than the time taken for the establishment of the said phenomenon.

- HOWARD B. BRIGGS.

WARREN P. MASON. JOHN B. JOHNSON.

REFERENES CITED The following references are of record in the Fig. 1 is used, iilled with degassed electrical grade of 10 watts'per square centimeter of crystal face le ofA this patent: 55 Y UNITED STATES PA Date Number vName 2,233,992 Wyckoi Mar.I 4, 1941 2,025,041 Colton et al. Dec. 24, 1935 2,147,649 Haines- -...s Feb'. 21, 1939 

